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Remove white mold infected annuals

September 20, 2016

M. Grabowski, UMN Extension Educator

Zinnias killed by white mold. M. Grabowski, UMN Extension

White mold is a plant disease caused by the fungus, Sclerotinia sclerotiorum. This pathogen is capable of infecting
over 400 plant species. Flower garden favorites like zinnia, petunia, salvia, and
snap dragon are highly susceptible to white mold. In the vegetable garden, tomatoes, green beans, lettuce and cabbage can all be infected. Removal of infected plants is a critical management strategy for white mold. The pathogen can survive up to 8 years in specialized resting structures produced on infected plant material.

Tomato stem with fluffy white fungal growth on the outside and
black sclerotia on the inside. M. Grabowski, UMN Extension

Plants infected with white mold often wilt and die. The lower stems of these plants will be tan and dry. If the humidity is high, white fluffy clumps of fungal
growth may be seen on the stems. Gardeners may also see small, rough, black
structures that look like seeds or peppercorns forming along stems or inside of them. These are special resting structures, called sclerotia that are
created by the fungus. Sclerotia can survive in the soil for up to 8 years.
Each year the sclerotia produce tiny mushroom like structures that releases
spores and starts new infections.

Gardeners that are seeing white mold in their gardens now,
need to remove infected plants before sclerotia are dropped into the soil. The
entire plant should be removed as soon as possible. Infected plants can be composted
if the pile heats up to a minimum temperature of 148 F. Alternatively infected
plants can be deeply buried (6-12 inches below ground) in an area of the yard that will not be used for
flowers or vegetables in the future like a mulched are around trees or shrubs.